Light olefins, defined herein as ethylene and propylene, are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals vinyl chloride, ethylene oxide, ethyl benzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
In addition to cracking petroleum products, the petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate to olefin (OTO) reaction process. Specifically, in an OTO reaction process, an oxygenate contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins. When methanol is the oxygenate, the process is generally referred to as a methanol to olefin (MTO) reaction process. Methanol is a particularly preferred oxygenate for the synthesis of ethylene and/or propylene.
In a typical OTO reaction system, undesirable byproducts, such as carbon dioxide, may be formed through side reactions. U.S. Pat. No. 6,121,504 discloses a method of making olefin product from an oxygenate feed using molecular sieve catalysts. Water and other unwanted byproducts are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, a light product fraction is obtained which comprises the desired olefins, but also includes dimethyl ether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock.
In order to further process olefins, it is often necessary to reduce or remove undesirable byproducts that are present in the olefin composition. For example, U.S. Pat. No. 5,914,433 discloses a method of making an olefin composition, and a system for removing non-olefin byproducts such as CO2. A dewatered olefin composition is washed with caustic to remove CO2, and the washed olefin composition is dried to reduce water added as a result of the caustic wash.
Published U.S. Patent Application US 2003/0199722 A1 to Lattner et al., the entirety of which is incorporated herein by reference, discloses a method of removing acetaldehyde, CO2 and/or water from an ethylene and/or propylene containing stream. Specifically, acetaldehyde and C4+ olefins are substantially removed from the ethylene and/or propylene containing stream. The stream is then acid gas treated.
Additional processes for removing undesirable components from olefin streams are sought. In particular, processes are sought for removing oxygenated hydrocarbons, particularly CO2, down to the ppm level in olefin product streams without removing significant amounts of olefin.